I have a barebottom tank and my fish produce crap like nobodys business so I do a waterchange weekly mostly to siphon all the fish poop out and various other detritus. I do 15 gallons mostly because I know where the 15 gallon level is in my garbage bucket and was too lazy to do any less. It only takes about 10 gallons to get all the detritus out and suck out any other crud off the rock etc. I suppose I could do it every second week but I hate to see it build up in there and I have a very watery green thumb when it comes to growing algae
